A gas boiler for producing hot water normally comprises a gas burner, and at least one heat exchanger through which combustion fumes and water flow. Some types of gas boilers, known as condensation boilers, condense the steam in the combustion fumes and transfer the latent heat in the fumes to the water. Condensation boilers are further divided into a first type, equipped with a first exchanger close to the burner, and a second exchanger for simply condensing the fumes; and a second type, equipped with only one heat exchanger which provides solely for thermal exchange along a first portion, and for both thermal exchange and fume condensation along a second portion. Condensation or dual-function exchangers of the above type normally comprise a casing extending along a first axis and through which combustion fumes flow; and a tube along which water flows, and which extends along a second axis and coils about the first axis to form a succession of turns. The combustion fumes flow over and between the turns to transfer heat to the water flowing along the tube.
EP 0 678 186 discloses a heat exchanger for a gas boiler for producing hot water. The heat exchanger comprises a casing extending along a first axis and through which combustion fumes flow; a tube forming a plurality of tube sections along which water flows; said tube sections being arranged inside said casing so as to forms gaps between adjacent tube sections; guiding means for guiding said fumes trough said gaps; and bosses for spacing adjacent tube sections.
Each tube section is provided with a cross section delimited by two parallel, opposite, flat walls. Bosses protrude from one of said flat walls for abutting a flat wall without bosses of an adjacent tube section and forming the above mentioned gaps between adjacent tube sections.
Even though the above described heat exchanger is provided with integrally made spacers, a rather expensive and time-consuming hydro-forming process is needed to form bosses in tube sections. The hydro-forming process is performed by a press that squeezes the tube sections between dies in order to form the flat walls and, at the same time, forms the bosses by injecting inside the tube sections a fluid under high pressure. It follows that hydro-forming process lacks flexibility because a modification of the distributions pitch or the height of the bosses requires different dies.
In addition to that, the process is not extremely accurate and small gaps cannot be formed by embossed tube sections.